The leader of Lebanon's Hezbollah has declared that the country's prime minister is currently detained in Saudi Arabia and that his "forced" resignation is unconstitutional because it was done "under pressure".

Speaking in Beirut on Friday, Hassan Nasrallah said he was sure that Saad Hariri was forced to resign as part of what he called Saudi Arabia's policy of stoking sectarian tensions in Lebanon.

Nasrallah said Hariri is being prevented by Saudi officials from returning to Lebanon, which is why "we deem the resignation of Hariri illegal and invalid".

"All of a sudden, out of nowhere, Saudi Arabia called the prime minister on urgent matter without his aide or advisers, and was forced to tender his resignation, and to read the resignation statement written by them," Nasrallah said, as he accused Riyadh of "blunt, unprecedented interference".

"We declare that the prime minister of Lebanon has not resigned," he said. "Saad Hariri is our political opponent, but he is also our prime minister."

Nasrallah also said "Lebanon had enjoyed unprecedented stability over the past year", and appealed for unity throughout the country.

He said US President Donald Trump must have known of the plans to force Hariri's resignation.

"No to proxy conflicts'

In a statement on Friday issued following Nasrallah's televised address, US Secretary of State Rex Tillerson warned against using Lebanon "as a venue for proxy conflicts".

He urged "all parties both within Lebanon and outside to respect the integrity and independence of Lebanon's legitimate national institutions".

"The United States supports the stability of Lebanon and is opposed to any actions that could threaten that stability."

Earlier, Heather Nauert, state department spokesperson, said a US diplomat met Saad Hariri in Riyadh, but refused to comment on where the meeting took place or to elaborate on Hariri's status.

"We have seen him. In terms of the conditions of him being held or the conversations between Saudi Arabia and Prime Minister Hariri, I would have to refer you to the government of Saudi Arabia and also to Mr Hariri's office."

Nauert said Hariri's resignation was an "internal matter that we couldn't comment on".

Separately, Russia's ambassador to Lebanon, Alexander Zasypkin, threatened on Thursday to refer Hariri's case to the UN Security Council if the "ambiguity" continues.

"The issue of Hariri's return to the country concerns the sovereign rights of Lebanon," Zasypkin said in an interview with Lebanese channel LBC.

Lebanese officials have said Hariri is likely to be under either house arrest or in temporary detention in Riyadh.

His resignation on November 4 came on the same day that dozens of Saudi princes, senior ministers, businessmen were arrested in a purge carried out by a new anti-corruption committee led by Crown Prince Mohammed bin Salman.

Also on Friday, France's foreign ministry said it wanted Hariri to be free of his movements and fully able to play an essential role in his country.

"As the minister said, we wish that Saad al-Hariri has all his freedom of movement and be fully able to play the essential role that is his in Lebanon," French deputy foreign ministry spokesman Alexandre Georgini said, refering to an earlier statement by Foreign Minister Jean-Yves Le Drian.

Georgini said France's ambassador to Saudi Arabia had also visited Hariri at his residence.